We are undertaking a major effort to elucidate the three dimensional structure of the protein human chorionic gonadotropin via x-ray crystallography and NMR spectroscopy. For the latter technique, gonadotropin is being produced with a high level of substitution of 13C and 15N. Mass spectrometry provides an accurate means of assessing the levels of incorporation of these isotopes into the protein. The determinations are being made at the level of the individual amino acids, selected peptides, as well as the intact protein. A paper describing the latter work has been submitted for publication.